At its inception radio telephony was designed, and used for, voice communications. As the consumer electronics industry continued to mature, and the capabilities of processors increased, more devices became available to use wireless transfer of data and more applications became available that operate based on such transferred data. Of particular note are the Internet and local area networks (LANs). These two innovations allowed multiple users and multiple devices to communicate and exchange data between different devices and device types. With the advent of these devices and capabilities, users (both business and residential) found the need to transmit data, as well as voice, from mobile locations.
The infrastructure and networks which support this voice and data transfer have likewise evolved. Limited data applications, such as text messaging, were introduced into the so-called “2G” systems, such as the Global System for Mobile (GSM) communications. Packet data over radio communication systems became more usable in GSM with the addition of the General Packet Radio Services (GPRS). 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users.
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the so-called 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radio communications in the decades to come. Among other things of note with regard to LTE systems is that they will provide for downlink communications (i.e., the transmission direction from the network to the mobile terminal) using orthogonal frequency division multiplexing (OFDM) as a transmission format and will provide for uplink communications (i.e., the transmission direction from the mobile terminal to the network) using single carrier frequency division multiple access (FDMA).
The capability to identify a user's geographical location in the network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by the application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, e.g., Federal Communication Commission's (FCC's) regulatory requirements for E911 services in the United States.
In many environments, the position of a user terminal can be accurately estimated by using positioning methods based on GPS (Global Positioning System). Nowadays networks have also often a possibility to assist UEs in order to enable the terminal to perform measurements at much lower receiver sensitivity level and improve GPS cold start or start up performance (Assisted-GPS positioning, or A-GPS). GPS or A-GPS receivers, however, may be not necessarily available in all wireless terminals. Furthermore, GPS is known to often fail in indoor environments and urban canyons due to lack of satellite coverage. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), is therefore being standardized by 3GPP.
With OTDOA, a terminal measures the timing differences for downlink reference signals received from multiple distinct locations. For each (measured) neighbor cell, the UE measures Reference Signal Time Difference (RSTD) which is the relative timing difference between neighbor cell and the reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations (BSs) with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias. In order to solve for position, precise knowledge of the transmitter locations and transmit timing offset is needed. Position calculation can be conducted, for example, by a positioning server (Evolved Serving Mobile Location Center or E-SMLC in LTE) or UE. The former approach corresponds to the UE-assisted positioning mode, whilst the latter corresponds to the UE-based positioning mode.
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning (positioning reference signals, or PRS) have been introduced and low-interference positioning subframes have been specified in 3GPP. PRS are transmitted with a pre-defined periodicity of 160, 320, 640 and 1280 ms. PRS are transmitted from one antenna port (R6) according to a pre-defined pattern as described, for example, in the standard specification 3GPP TS 36.211, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation, the disclosure of which is incorporated here by reference.
A frequency shift, which is a function of a physical cell identity (PCI), can be applied to the specified PRS patterns to generate orthogonal patterns and modelling the effective frequency reuse of six, which makes it possible to significantly reduce neighbor cell interference on the measured PRS and thus improve positioning measurements. Since, for OTDOA positioning, PRS signals from multiple distinct locations need to be measured, the UE receiver may have to deal with PRS signals that are much weaker than those received from the serving cell. Furthermore, without the approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern, the UE would need to do signal search within a large window, which would impact the time and accuracy of the measurements as well as the UE complexity. To facilitate UE measurements, the network transmits assistance data to the UE, which includes, among others things, a neighbor cell list containing physical cell identity (PCIs) of neighbor cells, the number of consecutive downlink subframes, PRS transmission bandwidth, etc.
To facilitate inter-frequency positioning measurements, when a neighbor cell in the assistance data is not on the UE's serving frequency, E-UTRA Absolute Radio Frequency Channel Number (EARFCN) for this cell is also signalled. The 3GPP standard specifies the positioning neighbor cell lists comprising up to 24 neighbor cells per carrier frequency. These are the cells for which the network signals the assistance data.
In LTE OTDOA, the UE measures Reference Signal Time Difference (RSTD) which has been defined in the standard document 3GPP TS 36.214, Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer measurements, the disclosure of which is incorporated here by reference. The measurements are specified for both intra-frequency and inter-frequency and conducted in the RRC_CONNECTED state (see Table 1 below).
TABLE 13GPP RSTD measurement definitionDefinitionThe relative timing difference between the neighbor cell jand the reference cell i, defined as TSubframeRxj −TSubframeRxi, where: TSubframeRxj is the time when the UEreceives the start of one subframe from cell j TSubframeRxiis the time when the UE receives the corresponding startof one subframe from cell i that is closest in time to thesubframe received from cell j. The reference point for theobserved subframe time difference shall be the antennaconnector of the UE.Applicable forRRC_CONNECTEDintra-frequencyRRC_CONNECTED inter-frequency
The inter-frequency measurements, including RSTD, are conducted during periodic inter-frequency measurement gaps which are configured in such a way that each gap starts at an SFN (System Frame Number) and subframe meeting the following condition:SFN mod T=FLOOR(gapOffset/10);subframe=gapOffset mod 10;
with T=MGRP/10, where MGRP stands for “measurement gap repetition period.” E-UTRAN provides a single measurement gap pattern with constant gap duration for concurrent monitoring of all frequency layers and RATs. Two configurations are supported by the UE, with MGRP of 40 and 80 ms, both with the measurement gap length of 6 ms. In practice, due to the switching time, this leaves less than 6 but at least 5 full subframes for measurements within each such measurement gap. An RSTD measurement is considered to be an inter-frequency measurement when the reference cell and/or the neighbor cell belong to the frequency, which is different than the serving cell carrier frequency.
In LTE, measurement gaps are configured by the network to enable measurements on the other LTE frequencies and/or other RATs. The gap configuration is signaled to the UE over RRC protocol as part of the measurement configuration. In multi-carrier LTE, the inter-frequency measurement gaps are so far intended mainly for performing mobility measurements such as Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ). These measurement gaps enable UEs to perform measurements over the synchronization signals, i.e. primary synchronization signals (PSS) and secondary synchronization signals (SSS), and cell-specific reference signals (CRS) to enable inter-frequency handover and enhance system performance.
Synchronization signals are transmitted over 62 resource elements in the center of the allocated bandwidth in subframe 0 and 5. The PSS is transmitted in the last OFDM symbol and the SSS is transmitted in the second last OFDM symbol of the first slot of a subframe. CRS symbols are transmitted every subframe and over the entire bandwidth according to one of the standardized time-frequency pattern. Different cells can use 6 different shifts in frequency and 504 different signals exist. With 2 TX antennas, the effective reuse for CRS is three.
As can be seen from the above, both synchronization signals and CRS are transmitted relatively often, although PSS and SSS are transmitted less frequently than CRS. This leaves enough freedom when deciding the exact timing of measurement gaps so that a gap could cover enough symbols with the signals of interest, i.e. PSS/SSS and/or CRS. With a 6 ms measurement gap, at most 2 SSS and 2 PSS symbols can be received with very precise timing, which may be not very realistic, while capturing 1 SSS and 1 PSS symbols is possible without any timing restriction on the measurement gaps since the minimum required effective measurement time is 5 ms on average.
In the current technologies, and due to the fact that there is typically only a single receiver in most UEs, the use of measurement gaps is necessary to conduct inter-frequency measurements. In the prior solutions, the exact time slots when the inter-frequency measurements are performed are typically decided by the network based on some criteria. For instance when RSRP and/or RSRQ from the serving cell fall below a threshold or there are no good intra-frequency candidate cells for performing handover, such inter-frequency measurements can be performed.
When measuring PSS, SSS and/or CRS on another frequency (i.e. mobility measurements), the exact timing of inter-frequency measurement gaps is essentially unrestricted. However, a number of issues arise associated with inter-frequency measurements.
One such issue is that the configured measurement gaps may be misaligned with positioning occasions. The standardized PRS have a minimum periodicity of 160 ms, which is far beyond the maximum length of an inter-frequency measurement gap. The maximum periodicity of the PRS occasion is 1280 ms. With no restriction on the exact timing of the measurement gaps, it may happen with a high probability that the other-frequency PRS will always be missed, although the UE will trust the assistance data received from the network and will try to measure PRS for the specified neighbor cell on the specified frequency. This in turn, may cause a number of problems. For example, in the case of false detection, a poor or inaccurate measurement may be used for position estimation by the UE or by the network. Additionally, the useful part of the overall measurement time decreases since the time spent for measuring on the other frequency and searching for a PRS signal which is not there (i.e. is outside the measurement gap) is essentially lost, and could have been spent for measuring other cells. Moreover, the UE cannot trust a network which provides unreliable assistance data and degrades the overall positioning performance and the UE may thus also choose to never conduct inter-frequency positioning measurements, which makes signaling of this information useless and consumes the network resources inefficiently.
Another issue which may arise is that the effective measurement length does not fully cover the maximum length of a positioning occasion. This is because up to 6 consecutive subframes can be configured for each positioning occasion, whilst the effective measurement period is shorter than 6 ms.
Yet another problem with conventional measurement approaches is that measurement gaps occur more often than positioning occasions. The periodicity of inter-frequency measurement is either 40 ms or 80 ms, while the minimum PRS periodicity is 160 ms. This implies that roughly at most only one-quarter (with 40 ms gap period) or one-half (with 80 ms gap period) of the total measurement gap time is usefully spent for PRS measurements, while the other time is lost, which time could have been spent for measuring cells on other frequencies.
One solution to these problems with inter-frequency measurements would be to limit positioning measurements only to intra-frequency. However, such a solution is undesirable since it underutilizes the available technology, e.g., including a UEs' capability of measuring on another frequency; the RSTD measurement standardized by RAN1 for both intra- and inter-frequency, the already agreed signaling support for inter-frequency measurements (e.g., the EARFCN indicator); the information available in the network and the possibility in the network to make the inter-frequency measurements working also for positioning; the multi-layer network structure, where different layers could operate on different frequencies which is also more efficient from the interference coordination point of view; the multi-carrier network operation which has been successfully used in earlier generations, etc., and that interference on different carrier frequency layers may be different and in some scenarios it may be easier to find sufficient cells on the inter-frequency for the positioning measurements.
Accordingly, it would be desirable to provide methods, devices, systems and software that would avoid the afore-described problems and drawbacks and enable inter-frequency measurements, e.g., for positioning, in measurement gaps.